Reactive oxygen species (ROS) have been implicated in cancer development for many years. A prime example where ROS are strongly implicated is the model system where feeding a choline deficiency (CD) diet to rats leads to hepatocellular carcinoma (HCC) development, i.e., in the complete absence of exposure to any exogenous known carcinogen. Utilizing this model, we have made novel observations that make it possible to link ROS with key signal transduction pathways that have been shown to be fundamental in cancer initiation and development. For the first time we have shown that mitochondria from CD-livers are changed such that they mediate a significantly higher yield of H202 production. Additionally, for the first time we have shown that PBN (a- phenyl-tent-butyl nitrone), a nitrone-based free radical trap, significantly reduces preneoplastic nodule development as well as inhibits hepatocellular carcinoma (HCC) formation at very low levels of the compound. PBN is the most potent anti-carcinogen ever studied in this model. To explain these observations we postulate that the CD-regimen mediates changes in mitochondrial membranes such that they produce enhanced levels of H2O2 and that PBN significantly inhibits the excess H202 production by acting at Complex 1. We further postulate that excess H202 causes an enhanced inactivation of the PTEN tumor suppressor protein, which causes a loss of its phosphatase activity and thereby mediates a shift toward the activation of the AKT-kinase pathway resulting in a decrease in apoptosis medicated processes but an increase in oncogenic events. We further propose that the cells in preneoplastic nodules that develop in CD-livers are predisposed toward oncogenesis (as opposed to apoptosis) because of the action of excess H202 and certain growth factors (most likely TGFbeta1) and that PBN alters these processes through both its inhibition of excess H202 production and also by suppression of enhanced signal transduction processes. We propose that PBN acts to cause the prenoplastic nodule cells to become predisposed toward apoptic processes thus leading to inhibition of tumor development. To test this hypothesis we have proposed 3 specific aims. Briefly they are: A) We will determine the nature of the alterations in mitochondrial membranes that cause them to produce excess H202. B) We will focus on the mechanisms of how H2O2 acts as a signaling molecule and the action of PBN in altering signal transduction processes in cell models. C) We will determine if PBN (or its metabolite 4-OH-PBN) inhibits CD-mediated HCC development and ascertain if these compounds increase apoptosis in preneoplastic cells in this model.